1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine.
2. Description of the Related Art
In a known engine, a three way catalyst which is able to purify HC, CO and NO.sub.x at the same time is arranged in the exhaust passage. A good purifying operation by the three way catalyst can be obtained when the temperature of the three way catalyst is high, but the purifying operation by the three way catalyst cannot be expected when the temperature of the three way catalyst becomes low. Therefore, in a known engine, when the temperature of the three way catalyst becomes low, the air-fuel ratio of air-fuel mixture fed into the engine cylinders is made rich, and at the same time, a secondary air is fed into the exhaust passage upstream of the three way catalyst to promptly raise the temperature of the three way catalyst (Japanese Unexamined Patent Publication No. 61-58912). In this engine, a large amount of unburned HC and CO, which are discharged from the engine when the air-fuel mixture is made rich, are oxidized by a secondary air, and the temperature of the three way catalyst is caused to rise by the heat produced by the oxidizing reaction.
In this way, if a secondary air is fed into the exhaust passage, HC and CO are oxidized. At this time, however, the reducing operation of NO.sub.x is not carried out and, thus, in this engine, a problem arises in that a large amount of NO.sub.x is discharged into the outside air.